Biodiesel is a long chained alkyl ester produced by the esterification or transesterification of triglycerides. Its color ranges from water-white to a straw-like yellow. Typical feedstocks include vegetable oils (soy and rapeseed are most popular), animal tallows and yellow grease. The reaction is summarized below:

Typically methanol is used as the alcohol and sodium hydroxide or sodium methylate as the catalyst. Currently, in the U.S., the majority of biodiesel is produced using batch technology. The oil is preheated to around 150 degrees F. (about 65.5 degrees C.) and the alcohol and catalyst are added. The mixture is stirred for around 1-2 hours after which agitation is stopped. The reaction proceeds and the glycerine co-product, which is immiscible with the alkyl ester, falls to the bottom of the reactor. Once the reaction has proceeded to the required point, the glycerine is decanted off and the biodiesel top phase is further purified.
Often the oil to be processed contains appreciable amounts of free fatty acids (FFAs). In this case the FFAs react with the alkali catalyst to produce soaps. These soaps separate out with the glycerine phase, the impact is to reduce the process yield and to reduce the purity of the glycerine phase.
The U.S. biodiesel industry is currently in its infancy and annual production in 2008 is estimated to be around 260 million gallons (about 9.84×108 liters). However, it is thought that this figure may reach 2.5 billion gallons (about 9.46×109 liters) in the short term future. The majority of the biodiesel is blended with petroleum derived diesel (“petrodiesel”) in a ratio of 20% biodiesel to 80% petrodiesel, the fuel is known as B20. B20 is a direct replacement to diesel fuel and generally approved by the automobile and agriculture industry and as such does not invalidate engine warranties.
Fixed bed gasifier and combustion systems typically require that the fuel utilized is of a sufficient size to be supported by a grate. Typically this requires that a biomass have at least one characteristic length be in the range of about ⅛″ to 2″ (about 0.31 to 5.08 cm). For some carbon-containing feedstocks, including coal and wood the solid can be ground or chipped and graded such that this requirement is met. However, many feedstocks have typical characteristic lengths less than this requirement. In this case pelletization or briquetting techniques can be used to increase the typical characteristic lengths of the feedstock, thus making the feedstock suitable for fixed bed-based applications. They typically apply pressure and heat to the solid to aid in the compaction.
Techniques to convert different solids into pellets or briquettes are well known. Techniques have been developed to compact non-organic powders including dust, metal chippings and ionic salts. Techniques have also been developed to produce pellets from organic materials including but limited to, sawdust, alfalfa, corn silage, dried distillers grains, bagasses and animal waste. The pellets produced through pressure and heat alone are often not of sufficient strength to survive mechanical feeding and crushing while in the fixed bed or the gasification or combustion device. The pellets produced through pressure and heat alone are often not of sufficient strength to survive mechanical feeding, thus the particle size distribution may change during feeding and a substantial volume of fine material may be formed. The formation of fines can vastly change the combustion or gasification characteristics as well as modifying the bed hydrodynamics. Also, pellets can become crushed while in the fixed bed or the gasification or combustion device, again resulting in fines formation. Components with the solids often polymerize to aid in the formation of a stable solid structure. Sometimes the solid is more difficult to process and in these cases a binding agent is often added. These binding agents act to “adhere” the smaller particles to produce the larger solid. In these cases the addition of a glycerine-rich stream would enhance the tensile strength of the final pellet.